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Alpha Centauri
' Alpha Centauri' (α Centauri, α Cen; also known as Rigil Kentaurus, Rigil Kent, or Toliman) a brightest with the possible exception of Proxima Centauri, which may or may not be part of it, the nearest star system to the Sun. α Cen (also known as Rigil Kentaurus, “the Centaur's foot” in Arabic) is the third brightest star in the night sky, after Sirius and Canopus, but lies so far south that it is visible only from latitudes below 25° N. Its two bright components, A and B, consist of a yellow G star, similar to the Sun but about 200 million years older, and an orange K star of the same age as its brighter companion, moving around each other every 79.9 years in a highly elongated orbit (eccentricity 0.519) with a mean separation of 23.7 AU—a bit more than the distance between Uranus and the Sun. Their binary nature was first observed by Nicolas Lacaille in 1752. Both lie 4.36 light-years (41 trillion km) away. No planets have been found in the system to date. Theoretical planets Early computer-generated models of planetary formation predicted the existence of terrestrial planets around both Alpha Centauri A and B, but most recent numerical investigations have shown that the gravitational pull of the companion star renders the accretion of planets very difficult. Despite these difficulties, given the similarities to the Sun in spectral types, star type, age and probable stability of the orbits, it has been suggested that this stellar system could hold one of the best possibilities for harbouring extraterrestrial life on a potential planet. Some astronomers speculated that any possible terrestrial planets in the Alpha Centauri system may be bone dry or lack significant atmospheres. In the Solar System both Jupiter and Saturn were probably crucial in perturbing comets into the inner Solar System. Here the comets provided the inner planets with their own source of water and various other ices86 but Proxima Centauri may have influenced the planetary disk as the Alpha Centauri system was forming enriching the area round Alpha Centauri A and B with volatile materials. This would be discounted if, for example, Alpha Centauri B happened to have giant gas planets orbiting Alpha Centauri A (or conversely, Alpha Centauri A for Alpha Centauri B), or if the stars B and A themselves were able to successfully perturb comets into each other's inner system like Jupiter and Saturn presumably have done here. As comets probably also reside in some huge Oort Cloud located to the outer regions of stellar systems, when they are influenced gravitationally by either the giant gas planets or disruptions by passing nearby stars, many of these comets then travel sun-wards. As yet, there is no direct evidence of the existence of such an Oort Cloud around Alpha Centauri AB, and theoretically this may have been totally destroyed during the system's formation. To be in the star's habitable zone, any suspected Earth-like planet around Alpha Centauri A would have to be placed about 1.25 AU away – about halfway between the distances of Earth's orbit and Mars's orbit in the Solar System – so as to have similar planetary temperatures and conditions for liquid water to exist. For the slightly less luminous and cooler Alpha Centauri B, this distance would be closer to its star at about 0.7 AU (100 million km), being about the distance that Venus is from the Sun. With the goal of finding evidence of such planets, both Proxima Centauri and Alpha Centauri AB were among the listed "Tier 1" target stars for NASA's Space Interferometry Mission (SIM). Detecting planets as small as three Earth-masses or smaller within two Astronomical Units of a "Tier 1" target would have been possible with this new instrument.However, the SIM mission was cancelled due to financial issues in 2010. Glyphus Project The artificial planet Glyphus has be constructed in 2089 by the United Nations Astronomical Administration . Category:Stars Category:Binary Stars